1. Field of the Invention
The present invention relates to a resinous composition for dental use. More precisely, the invention relates to a resinous composition suitable for denture bases, denture rebases, orthodontical bases, denture repairs, mouth pieces and temporary crowns.
2. Description of the Background
In ordinary dental treatment in prosthodontics and orthodontics, resious compositions are used for prostheses for denture bases and orthodontical bases. For producing those prostheses, mainly employed is a method of mixing a powdery component consisting essentially of polymethyl methacrylate and a liquid component consisting essentially of methylmethacrylate, followed by polymerizing and curing the resulting mixture. However, this method has a problem in that the mixture must be kept as such for a while until it becomes plastic enough to be filled into a gypsum mold. In addition, since its viscosity increases with a lapse of time, the mixture, if left longer than a predetermined period of time, becomes too hard to use, and the application time of the mixture is limited anyhow. Further there is another problem in that the mixture involves bubbles during mixing the powdery component and the liquid component together and bubbles in the resultant cured product decrease the mechanical strength of the cured product and the impact resistance of the same.
To improve the impact resistance of the cured product, a resinous composition for dental use comprising a powdery elastomer was proposed (see Japanese Patent Laid Open (JP-A) No. Hei-1-275509). However, the proposed composition has poor dispersion of the powdery elastomer in it and therefore the impact resistance of the cured product is still not satisfactory. In addition, as the powdery elastomer is formed to be amorphous by grinding, the powdery elastomer is not satisfactorily miscible with resinous composition for dental use, and the mixing of the components is troublesome. Moreover, the elastomer, due to its component, often causes discoloration of the cured product while the product is set in the mouth for a long period of time, and the appearance of the cured product becomes aesthetically poor.
Also a composition for dental use comprising a rubber-graft copolymer, a (meth)acrylate monomer and a polymerization initiator was proposed (see JP-A Hei-3-63205). However, the proposed composition still has a problem in that since the rubber-graft polymer and the (meth)acrylate monomer are mixed by blending, the rubber-graft copolymer particles are aggregated in the composition and are poorly dispersed therein though the particles are miscible with (meth)acrylate monomer to some extent. Therefore, the impact resistance of the composition for dental use and for denture bases still is not satisfactory. In addition, the discoloration resistance of the cured product is not improved at all, and the appearance of the cured product is still aesthetically poor.
To improve the dispersibility of the rubber-graft copolymer in the composition, a different composition was proposed for denture bases comprising methyl methacrylate polymer particles containing an inner layer of a rubber-graft copolymer, a (meth)acrylate monomer and a polymerization initiator (see JP-A Hei-6-57157). Though the dispersibility of the above-mentioned polymer particles in the composition was improved, the proposed composition still has some problems in that the cured product of the composition is discolored with hot water and its mechanical strength decreases while it absorbs water because of a suspending and dispersing agent required in producing the polymer particles.
On the other hand, proposed was still another composition for denture bases comprising methyl methacrylate polymer particles coated with a rubber-graft copolymer, a (meth)acrylate monomer and an organic peroxide (see JP-A Hei-6-247824). The above-mentioned methyl methacrylate polymer particles show improved dispersibility in the composition. However, the composition still has problems in that the cured product of the composition is discolored with hot water to damage the aesthetic appearance of the denture bases and its mechanical strength decreases while it absorbs water, due to a coagulant used in producing the polymer particles.
As has been mentioned hereinabove, the conventional technique of improving the dispersibility of rubber-graft copolymer in resinous composition resulted in the improvement in the impact resistance of the cured product for dental use or denture bases. However, since the rubber-graft copolymer contains stabilizer or coagulant in large quantities that are used in producing the polymer, the resinous composition for dental use comprising the rubber-graft copolymer has problems in that its mechanical strength decreases while the cured product is used for a long period of time in the mouth and it absorbs water, and in that the cured product is discolored with edible dyes and hot water.
Further proposed was a polymerizable composition comprising polyfunctional crosslinking monomer or oligomer, crosslinked polymer and rubber-modified polymer (see JP-A Hei-7-286018), which has easy handling of the composition and provides cured products having improved impact resistance. However, it was realized that the above composition has a problem in that the impact resistance of the cured product is still unsatisfactory since it contains crosslinked polymer as the essential component for improving the handling of the composition. In addition, no improvement is attained in preventing the discoloration of the cured product of the composition.